1. Field of the Invention
The present invention relates to an electronic circuit substrate having film devices such as a conductive circuit provided on the surface thereof and which is excellent in reliability.
2. Related Art Statement
Various electronic circuit substrates have recently been known and put to practical use. For example, electronic circuit substrates using glass/epoxy resin composite, alumina sintered body, mullite sintered body or the like have been proposed and practically used. As a method for the promotion of high integration, a method of directly mounting a silicon integrated circuit and the like onto the substrate has been investigated.
However, since the glass/epoxy resin composite is largely different from the silicon integrated circuit in the thermal expansion coefficient, only a very small silicon integrated circuit can be directly mounted on the substrate. In addition, the substrate of the glass/epoxy resin composite is liable to change its size at a circuit formation step, so that it is particularly difficult to apply this substrate to the formation of fine and precise circuits.
An alumina sintered body and a mullite sintered body have a high hardness and, hence, a poor machinability. Therefore, if machining for forming through holes or the like is necessary, after a green shaped body is machined, the machined body is fired. By this method, however, since it is difficult to make the shrinkage uniform at the firing step, it is especially difficult to produce substrates which are required to have a high dimensional accuracy or a large size.
To deal with these problems, Japanese Patent Laid-Open Nos. 287190/1986 and 82689/1989, and U.S. Pat. No. 4,882,455 disclose a substrate of a porous ceramic sintered body with the pores filled with a resin.
In this substrate, the thermal expansion thereof is varied in conformity with the thermal expansion of a part mounted thereon such as a silicon integrated circuit by varying the porosity of the ceramic material. The thermal expansion of this substrate is small, so that the substrate is excellent in dimensional stability. In addition, since machining of the substrate is easy, it can cope with the tendency toward the increase in the size and reduction in the weight of integrated circuits.
For the purpose of high integration, multi-layer electronic circuit substrates produced by laminating a plurality of electronic circuit substrates have recently been widely used. Electronic circuit substrates having film devices such as a resistor and a capacitor provided on the circuit in the form of films in place of chips have been developed. By forming such film devices such as a conductive circuit, resistor and capacitor, it is possible to reduce in the size and weight of an electronic circuit substrate.
However, an electronic circuit substrate produced by forming film devices on the above-described resin-filled porous ceramic substrate is poor in reliability.
More specifically, since the film devices formed on the resin-filled porous ceramic substrate are situated on the resin, the behavior of the resin greatly influences the film devices. For example, the resistance and the capacitance of the film devices inconveniently greatly varies at a high humidity or a high temperature. In addition, since the film devices are formed on the resin surface, the film devices have the possibility of the separation from the resin-filled porous ceramic sintered body.